1. Field of the Invention
The present invention relates to a light diffusing film, a polarizing plate, a liquid crystal display device, and a production method of a light diffusing film.
2. Description of the Related Art
In recent years, a liquid crystal display device (LCD) has been widely used because of its thinness, lightweight and low power consumption. The liquid crystal display device contains a liquid crystal cell and a polarizing plate. The polarizing plate is usually composed of a protective film and a polarizing film and is obtained by dyeing a polarizing film formed from a polyvinyl alcohol film with iodine, stretching the film, and stacking a protective film on both surfaces thereof. In a transmission-type liquid crystal display device, this polarizing plate is attached to both sides of a liquid crystal cell and one or more optically compensatory sheets are sometimes further disposed thereon.
A surface light source is used in the transmission-type liquid crystal display device. As the surface light source, a backlight type where a uniform surface light source is obtained by placing a member having a light diffusing function or a light collecting function, such as diffuser sheet or prism sheet, to intervene between a liquid crystal cell and a light emission source is widely used, and a cold-cathode tube (CCFL) or LED is used as the light emitting source. Also, some known LCD are of a type where a light source is disposed in the edge part of a light guide plate and combined with a light diffusing sheet (light diffusing film), a prism sheet or the like to work as a surface light source (edge light type). In these types, the light diffusing sheet is generally used for converting a line light source or a point light source into a surface light source as described above.
Out of the constituent members for obtaining a surface light source, the light diffusing sheet is generally disposed between a backlight and a polarizing plate on the backlight side. By disposing the light diffusing sheet, reduction of luminance unevenness ascribable to a light source and surface uniformization of display characteristics are achieved and at the same time, luminance unevenness such as moire generated due to interference of incident light with a pixel in the liquid crystal cell can be suppressed.
In the conventional liquid crystal display device shown in FIG. 1 of JP-A-2006-208535 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), when a prism sheet is disposed between the liquid crystal cell and the backlight source, moire is generated due to interference of the prism sheet with a pixel in the liquid crystal cell. In order to prevent such moire, JP-A-2006-208535 discloses a method where in a liquid crystal display device using one or two prism sheets, a first diffuser sheet (sometimes referred to as a lower diffuser sheet) is disposed between a prism sheet and a light guide plate and a second diffuser sheet (sometimes referred to as an upper diffuser sheet) having a haze value of 60 to 80% is disposed between the prism sheet and a liquid crystal cell.
However, this method requires a second diffuser sheet, and light loss is produced due to reflection on the front or back surface of the new member. Also, in Patent Document 1, a light diffusing sheet having a haze value of 60 to 80% is used as the second diffuser sheet, but when a diffuser sheet having such a high haze value is used for light already transmitted through a prism sheet, the light causes backward scattering or wide-angle scattering and does not reach the front surface and therefore, the front luminance is reduced.
As a method for solving this problem, JP-A-8-068997 discloses a liquid crystal display device fabricated in the configuration above by omitting the second light diffusing sheet and using two prism sheets, wherein a predetermined angle is imparted to the groove direction of the prism sheet and the repeating direction of pixels in the liquid crystal cell and at the same time, those two prism sheets are disposed by arranging their groove directions to run at right angles to one another.
The present inventors have studied to remove a light diffusing sheet by using the method above and found that in this method, the moire observed at the front is surely reduced, but when viewed from a direction orthogonal to the groove of the prism sheet and tilted to the plane direction of the prism sheet from the normal line of the prism sheet, a stripe-like luminance unevenness attributable to the prism groove is observed in an oblique direction (oblique luminance unevenness), impairing the image quality.
The above-described second diffuser sheet is effective in eliminating such a stripe-like oblique luminance unevenness, but the problem of reduction in front luminance re-emerges.
On the other hand, with respect to preventing the interface reflection by the introduction of a second diffuser sheet, attempts are being recently made to reduce the thickness of the display device by decreasing the number of members of a liquid crystal display device or for realizing low power consumption, to decrease the number of fluorescent lamps used as the light source by preventing reflection on the interface of members and thereby effectively utilizing light. Following this trend, it is being studied to combine a light diffusing sheet and a backlight-side polarizing plate.
For example, JP-A-2000-075134 has proposed a light diffusing and polarizing plate where a light diffusing layer formed of a photocurable resin having dispersed and incorporated therein porous amorphous particles and spherical particles and imparted with surface unevenness giving a haze of 60% or more is stacked, and it is disclosed that thanks to this polarizing plate, even when a light diffusing sheet is removed, moire generated due to interference of a prism sheet with a pixel in the liquid crystal cell can be prevented.
However, this method uses a diffuser sheet having a high haze value and therefore, the problem of reduction in front luminance due to the above-described backward scattering or wide-angle scattering of light is unsolved.
Also, in the method above, a light diffusing layer is stacked by coating and curing a photocurable resin containing particles on a polarizing plate, and a step of stacking the light diffusing layer must be provided after producing a polarizing plate, which brings about a new problem that the productivity is low.
On the other hand, JP-A-2001-172403 has proposed a production method of a light diffusing film, comprising a step of casting a dope containing fine particles on a support, and it is disclosed that according to this method, a light diffusing film endowed with good surface quality and high productivity can be produced. Use of this method is successful to a certain extent in solving the problem of productivity, but since a light scattering technique by a high haze is fundamentally used for the reduction of luminance unevenness, the problem of reduction in front luminance is not yet solved.
Summarizing these, a light diffusing film ensuring that the front luminance is high, the moire control effect is excellent, luminance unevenness in the oblique direction or the like is not observed, a display device mounted with the film is excellent in the image quality, and the productivity is high, has been heretofore unknown, and a light diffusing film having these characteristics and a production method thereof are being demanded.